Single crystal silicon micromirror and array

ABSTRACT

A micromirror is fabricated in a substrate by defining a mirror platform on a first side of the substrate, defining an actuator structure corrected to the platform on a second side of the substrate, and then releasing the mirror platform for motion with the actuator. The actuator may be a comb drive structure having interdigitated movable finger electrodes connected to the mirror platform and stationary finger electrodes mounted on the substrate. The movable and stationary finger electrodes preferably are asymmetrical, and when activated, controllably move the mirror platform either horizontally or vertically with respect to the surface of the substrate.  
     The comb drive structure may be connected at one of its ends to a torsional support beam secured to the substrate, for torsional motion of the mirror platform with respect to the substrate. Alternatively, the comb drive may be connected at both ends to spaced torsional support beams for vertical motion of the platform with respect to the substrate. In the latter case, the actuator preferably includes spaced hinges to allow expansion of the actuator length.

[0001] The present application claims the benefit of U.S. ProvisionalApplication No. 60/176,492, filed Jan. 18, 2000, the disclosure of whichis hereby incorporated herein by reference.

[0002] This invention was made with Government support under Grant No.______ awarded by DARPA. The Government has certain rights in theinvention.

FIELD OF THE INVENTION

[0003] The present invention relates, in general, to micromirrors and toarrays of such micromirrors, and more particularly to a low voltage,single crystal silicon micromirror assembly having a high fill factor,and to methods of fabricating such micromirrors and arrays.

BACKGROUND OF THE INVENTION

[0004] Significant applications of microelectromechanical structures(MEMS) arise from the fact that the fabrication process for such devicesallows individual microactuators to be organized into massive arrayswhich cooperatively perform a macroscopic function. A prominent exampleof the application of MEMS to such arrays, in which interconnectedmicroactuators are used, are digital projection displays based ondigital micromirror devices (DMD). This technology has been used inseveral applications, including fiber optic crossbar switches, wavefront correctors used in adaptive optics and free space communication,optical beam steering, and variable optical gratings. The DMD technologyis based on a released polysilicon thin film and metal micromachiningprocess, and on flexible dielectric membrane fabrication using KOH wetsilicon chemistry.

[0005] Mirror surfaces can be produced on such flexible dielectricmembranes, and these have several advantages over conventionalpiezoelectrically-actuated mirrors. Fabrication is made considerablyeasier, because no discrete assembly is required, and the actuators forthe mirrors can be integrated on the same chip. This process utilizesexisting semiconductor fabrication technology to take advantage of batchfabrication processes so that the final cost is low. Performance isenhanced because the system is operated at low voltage and low power.Because the process permits a high actuator density, high spatialresolution is available, and the arrays are lightweight so that highfrequency operation is possible.

[0006] The biggest drawback of the deformable mirrors produced by thisthin film technology is a residual stress in the deposited thin films aswell as a stress interaction between the substrate films and reflectivemirror films. A polysilicon mirror deformation of 50 nm after therelease of the structure has been reported. This produces deviationsfrom the flatness required for a mirror surface, and although it isrelatively small compared to the thickness of the mirror and the overallmotion of the mirror that can be produced by actuators, such deviationsrepresent significant fractions of the wave length of visible light and,therefore, adversely affect the performance of the mirror. Because ofthis residual stress problem, the rectangular size of such a mirror islimited to about 200 micrometers or less on each side. To relieve someof this stress, the mirror surface is often fabricated with apertures,which also help in the release process, but the resulting surface of thethin film can be optically rough, although post-deposition treatmentscan minimize this surface roughness.

[0007] Another difficulty with such thin film devices is that they arenormally driven by capacitive forces, with the result that motion in thevertical, or z-direction, is based on parallel plate forces. As aresult, the deflection of the mirror is not controllable if the motionexceeds about one third of the initial parallel plate gap.

[0008] Prior mirror actuators normally include a spring system formotion in response to signals supplied to the capacitor plates, and thedesign of such spring systems plays an important role in lowering theoperating voltages and minimizing actuator area. A variety of springdesigns have been developed, some using flexible torsional hinges as thespring system, with the hinges being hidden underneath the mirrorstructure. Deformable micromirror arrays composed of a flexiblepolysilicon membrane supported by an underlying array of electrostaticparallel plate actuators can provide extremely high fill factors, in therange of 90%. However, where the thin membrane acts as the spring systemfor the mirror structure, up to 150 V may be required to achieve acenter deflection of 1 μm, due to the typical geometry of the membrane.Further, the deflection of the membrane may not be uniform, resulting innonuniform vertical motion across the mirror surface. Tests of arraysusing folded flexures attached to the moving mirrors produced an overalleffective spring constant in the z-direction that was less than that ofthe membrane system so that a considerably lower operating voltage wasachieved. However, both the flexures and the mirror had to be fabricatedin the same layer so that any area taken up by the flexures woulddirectly reduce the area of the array covered by the movable mirrorsurfaces. Thus, a low fill factor of only about 40% was achieved withthis design.

SUMMARY OF THE INVENTION

[0009] The foregoing difficulties are overcome, in accordance with thepresent invention, by a micron-scale, single crystal silicon (SCS)micromirror assembly including a mirror platform having an opticalsurface, which is optically flat and smooth, free of residual stress,and which is highly reflective after the deposition of a thin metallayer. The assembly also includes a high aspect ratio MEMS actuatorstructure which supports the mirror platform and produces enhancedmanipulation of the optical surfaces.

[0010] In accordance with a preferred form of the invention, asuspended, or released, drive actuator is fabricated in one surface of adouble polished wafer, with the drive actuator supporting, and beingprecisely aligned with a corresponding micromirror platform structurefabricated in the opposite surface of the wafer. The polished wafersurface in which the mirror platform is fabricated provides an opticallyflat mirror surface for receiving a reflective coating such as a thinmetal layer, or multiple layer thin films. The mirror assembly isfabricated in the wafer by a suitable process, such as the SingleCrystal Reactive Etch And Metallization process (SCREAM) processdescribed and illustrated in U.S. Pat. No. 5,426,070 to Shaw et al,issued Jun. 20, 1995, and is released from the wafer by a through waferetch. The actuator is connected to the back of the corresponding mirrorplatform by rigid mounting posts to transfer the motion of the actuatorto the mirror.

[0011] Micromirror assemblies may be fabricated in arrays of any desiredsize, utilizing known fabrication techniques, with the individualactuators being operable through individually addressable electricalconnections. Routine silicon patterning can be done on the opticallyflat, SCS mirror surfaces for various optical applications, and scalingup of the arrays may be done.

[0012] The actuator which supports the mirror platform preferablyutilizes an asymmetric comb finger design having interdigitatedstationary and movable fingers having high aspect ratios. Verticalmotion of the micromirror assembly with respect to the surface of thesubstrate is produced by applying a net electric field between adjacentfingers. By providing comb finger electrodes having different heightsthe range of motion limitations of other parallel plate actuatorconfigurations are avoided.

[0013] The MEMS single crystal silicon micromirror platform provided bythe present invention has sufficient thickness and rigidity to permitfabrication of features such as optical gratings on the platform and themirror assembly has an excellent structural rigidity, to provide auniform motion across the optical surface of each mirror upon operationof its corresponding actuator. By providing an array of mirror platformson one side of a wafer and providing the corresponding actuators,contact pads, address metal lines and related structural features on theother side of the wafer, a high fill factor can be attained for themirror array; that is, up to about 90% of the array surface is mirrored,with the remaining portion being taken up by the spaces between adjacentmirrors.

[0014] A MEMS micromirror assembly in accordance with the presentinvention is fabricated in a single crystal silicon (SCS) substrate orwafer using, in one embodiment, a two-mask process. The SCS wafer ispolished on its upper and lower surfaces and both surfaces are coveredby an oxide layer. If the mirror is to be a reflective metal, forexample, the bottom surface is coated with a metal layer such asaluminum, and thereafter a first mask defining a micromirror platform islithographically defined on the bottom surface. A trench surrounding theplatform is etched through the mask, through the aluminum, and partiallythrough the wafer. The mirror surface remains covered by the mask duringthe succeeding steps, so it is not damaged during formation of theactuator structure and release of the mirror platform.

[0015] Thereafter, an actuator, or micromirror drive structure, whichmay be in the form of a comb-type capacitive drive, is fabricated in thetop surface of the wafer. In this embodiment, the top surface oxidelayer is replaced by a second oxide layer in which is photolithicallydefined the actuator pattern in careful alignment with the previouslyformed micromirror platform using a second mask. The actuator pattern istransferred to the silicon wafer by etching, and in accordance with theSCREAM process, the walls of the resulting trenches are covered with aconformal oxide layer. The oxide is removed from the floor of thetrenches and an isotropic etch is used to release narrow actuator beamstructures. Another conformal oxide layer is applied, the floor oxide isagain removed, and the trenches are deepened. Thereafter, anotherisotropic etch is used to release wider actuator beam structures, andthe actuator is metallized to form adjacent capacitive drive electrodes.By selecting the relative widths of the beam structures, released beamsof varying heights and aspect ratios can be produced, so that anasymmetric comb-type drive is formed by interdigitated movable andstationary fingers of different heights.

[0016] Following formation of the actuator drive structure, the trenchin the bottom surface surrounding the mirror platform is etched throughthe wafer to release the micromirror.

[0017] In a second, three-mask embodiment, the actuator is fabricatedusing second and third masks, following fabrication of the micromirrorplatform on the bottom of the wafer using a first mask, as describedabove. In this embodiment, the oxide layer on the top surface of thewafer is patterned through the second mask to define an area whereselected portions of the actuator are to be fabricated; for example,where the fixed fingers of a comb-type actuator are to be located. Thispattern is transferred to the top surface oxide layer, which isselectively etched to reduce the thickness of the oxide layer. The thirdmask is then used to define the pattern of the entire actuatorstructure, and this pattern is transferred into the silicon by theSCREAM process. The structure is then released and metallized, toproduce released metallized beams with selected heights, as determinedby the thickness of the top surface oxide layer on each beam.Thereafter, the bottom trenches are etched through the wafer to releasethe micromirror structure, as previously described.

[0018] Multiple adjacent micromirrors with corresponding actuators maybe fabricated in a single wafer, using the foregoing process, to form anarray of MEMS micrometer-scale micromirrors which cooperate to produce amacro-scale mirrored surface. Each micromirror assembly in the array isindividually movable and controllable.

[0019] The micromirror platform fabricated in the bottom surface of thewafer may be of any desired shape, and thus may be generallyrectangular, is relatively thick so as to be sufficiently rigid tomaintain optical flatness when the platform is moved by its actuator andto permit fabrication of optical gratings on it, and has dimensions inthe micrometer scale. The platform is surrounded by a narrowthrough-wafer trench and by sufficient space to permit routing of metalconnector lines, and these separate the micromirror from a surroundingsubstrate or, in the case of an array, from adjacent micromirrors. Eachmicromirror platform is attached to, and supported by, a correspondingcontrollable actuator structure which is fabricated in the top surfaceof the wafer and supported by torsion bars or springs.

[0020] The actuator, in one form of the invention, includes a backbonestructure which may incorporate plural longitudinal, parallel, highaspect ratio beams extending parallel to the length of the micromirror,with multiple transverse beams interconnecting the longitudinal beams ina ladder-like structure.

[0021] In one embodiment of the invention, the backbone is connected forpivotal motion about a single high aspect ratio torsion bar which isperpendicular to the backbone, with opposite ends of the torsion barbeing anchored to the substrate. In a second embodiment, the backbone isconnected to two spaced high aspect ratio torsion bars, with a pair ofspaced hinges, each including two stress-relieving bars, being locatedin the backbone between the torsion bars to divide the backbone intothree segments. The outer ends of both torsion bars are anchored to thesurrounding substrate to support the backbone in a cavity in thesubstrate. The two hinges are transverse to, and are coplanar, with thebackbone and are generally parallel with the torsion bars. These hingespermit the center segment of the backbone to move uniformly in az-direction, perpendicular to the plane of the torsion bars and to theplane of the wafer surface, while allowing the two end segments to pivotaround their respective torsion bars to permit the out-of-plane motionof the center segment.

[0022] Connected to the backbone, and to the middle segment of thebackbone in the second embodiment, is a comb-type actuator consisting ofmultiple movable, high aspect ratio actuator fingers extendingperpendicularly from the backbone, and multiple stationary, high aspectratio fingers mounted on the surrounding substrate and interdigitatedwith the movable fingers. The individual fingers have high height towidth ratio to provide relative stiffness in the vertical direction, andthis may have widths on the order of 0.5-3 μm and heights on the orderof 5-100 μm or more. Integral with the backbone are vertical supportposts which extend downwardly through the wafer, the lower ends of theposts being connected to, and integral with, the mirror platform formedin the bottom surface of the wafer.

[0023] The movable actuator fingers connected to the backbone arefabricated as described above to have a different height than that ofthe interdigitated fixed fingers. Both sets of fingers are metal-coatedon their tops and sidewalls to provide actuator electrodes, and thedifference in the electrode heights produces a vertical asymmetry in theelectric field between the stationary and movable fingers, when apotential difference is applied across the fingers, as described in U.S.Pat. No. 6,000,280, issued Dec. 14, 1999. The asymmetric electric fieldis mainly due to the difference in the height of the metal covering ofthe adjacent fingers, in known manner. As described in the aforesaidpatent, the asymmetric electric field distribution results in anout-of-plane actuation force that causes the movable fingers, and thusthe backbone, to move in a vertical direction upon the application ofvoltages to the electrodes. In the first embodiment of the invention,where the backbone is connected to a single torsional bar, a voltageacross adjacent electrodes produces relative vertical motion of themovable fingers with respect to the stationary fingers, and this causesthe cantilevered backbone to pivot about the axis of the torsional bar.This torsional operation is converted into a pure z-direction motion inthe second embodiment by connecting both ends of the backbone totorsional bars and incorporating the stress-relieving hinges discussedabove, thereby allowing accurate vertical translation of the mirror outof the plane of the bottom surface of the substrate.

[0024] In accordance with the invention, the described mirror structuresmay be fabricated in arrays on a wafer, with any practical number ofmirrors being provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The foregoing, and additional objects, features and advantages ofthe invention will be apparent to those of skill in the art from thefollowing detailed description of preferred embodiments thereof, takenin conjunction with the accompanying drawings in which:

[0026]FIG. 1 is a diagrammatic perspective view, in partial section, ofa micromirror and actuator assembly constructed in accordance with thepresent invention;

[0027]FIG. 2 is an exploded view of the device of FIG. 1;

[0028]FIG. 3 is a bottom perspective view of the micromirror structureof FIG. 1;

[0029]FIG. 4 is a bottom perspective view of the micromirror device ofFIG. 1 illustrating its torsional motion;

[0030]FIGS. 5 and 6 are diagrammatic illustrations of electrostaticforces producing out-of-plane motion in mirror actuators;

[0031]FIG. 7 is a diagrammatic top plan view of a second embodiment of amicromirror actuator assembly producing vertical, or z-direction, motionutilizing the actuators of FIGS. 5 and 6;

[0032]FIG. 7A is a diagrammatic top perspective view of the actuatorassembly of FIG. 7;

[0033]FIG. 8 is a diagrammatic cross-sectional view of the device ofFIG. 7, taken along lines 8-8 and illustrating the z-direction motion ofthe device of FIG. 7;

[0034]FIG. 9 is a bottom perspective diagrammatic view of a verticalmotion micromirror of the type illustrated in FIG. 7;

[0035]FIG. 10 is a schematic illustration of a 6×6 array ofmicromirrors, illustrating address line routing;

[0036] FIGS. 11-17 provide a diagrammatic illustration of the steps of aprocess for fabricating a two-depth, large motion MEMS actuator assemblyin single crystal silicon; and

[0037] FIGS. 18-28 illustrate diagrammatically the steps for fabricatinga micromirror structure utilizing an alternate procedure for obtainingtwo-depth electrodes.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] Turning now to a more detailed description of the presentinvention, there is illustrated in FIG. 1 a diagrammatic, perspectiveview of a micromirror structure, or assembly, 10 suspended in asubstrate 12, which preferably is a single crystal silicon (SCS) wafer,in accordance with the present invention. The assembly is amicroelectromechanical structure (MEMS), preferably fabricated utilizingthe known SCREAM process and a through-the-wafer etching process,together with a conventional front-to-back alignment technique. Multiplemicromirror assemblies 10 can be fabricated in a single substrate toform a micromirror array, as will be described in greater detail below,but for convenience the structure and fabrication process will bedescribed herein in terms of a single micromirror assembly having amirror platform suspended from an actuator which, in turn, is suspendedwithin a cavity extending through the SCS substrate. Each micromirrorassembly 10 includes an actuator 14 such as a comb-type capacitiveactuator fabricated in a first (or top) surface 16 of the doublepolished substrate 12, and a micromirror platform 18 fabricated in asecond (or bottom) surface 20 of the substrate 12. The micromirrorplatform 18 is connected to the actuator 14 by one or more integral,rigid support posts, four of which are illustrated in FIG. 1 at 22, 24,26 and 28.

[0039] In the illustrated embodiment, the micromirror assembly 10includes a movable portion which is supported at one end by means of atorsional beam, or spring, 30 in cantilever fashion for pivotal motionabout beam 30 within a cavity 32 formed in the substrate. The beam 30extends across the cavity 32 and is secured at opposite ends to thecavity sidewall 34. The pivotal motion of the micromirror assembly 10 isillustrated in FIGS. 3 and 4, and is produced by the application of asuitable voltage across stationary fingers 40, mounted on the sidewall34, and interdigitated movable fingers 42, mounted on the movableportion of the assembly, which form the comb-type capacitive actuator14.

[0040] In a preferred form of the invention, the movable portion ofactuator 14 includes a horizontal backbone 44 parallel to the topsurface of the substrate and including two parallel beams 46 and 48, thebeams being held in spaced apart relation by transverse cross bars 50.The beams 46 and 48 have high aspect ratios; that is, the height of eachbeam is greater than its width to produce stiffness in the verticaldirection. The support posts 22, 24, 26 and 28 are secured to or areintegrally formed with the backbone and extend through the wafer to theplatform 18. The movable fingers 42 extend outwardly from, and arecoplanar with, the backbone 44 and also have high aspect ratios forstiffness in the vertical direction.

[0041] In a preferred fabrication process, the micromirror platform 18is formed in the bottom surface of the substrate. Then the actuator isfabricated in the top surface and is released for motion with respect tothe substrate, utilizing the SCREAM process, with the actuator beingprecisely aligned with the micromirror platform using a front to backaligner. Thereafter, the micromirror platform is released from thesubstrate by a through wafer etch from the bottom surface of thesubstrate. In the fabrication process, the movable and stationaryfingers 40 and 42 are electrically insulated and then are coated with analuminum layer to provide electrically conductive layers on opposedfinger surfaces. The silicon cores of the fingers and beams areconnected to each other, but the metal layers are physically separatedand electrically isolated from the silicon by a sidewall PECVD oxide, asknown in the SCREAM process, to provide opposed, spaced electrodes onadjacent fingers.

[0042] As described in the aforesaid U.S. Pat. No. 6,000,280, andillustrated in FIG. 5, upon application of a suitable potentialdifference across adjacent movable and stationary finger electrodes, anelectrostatic field 52 is produced between the adjacent fingers toproduce a vertical force, illustrated by arrow 54 in FIG. 5. Thisvertical force causes the movable fingers to move away from thestationary fingers, causing the micromirror assembly 10 to pivot abouttorsional beam 30, as illustrated in FIGS. 3 and 4.

[0043] Although the fingers 40 and 42 may be fabricated with the sameheight, in a preferred form of the invention the fixed and movablefingers have different effective heights, and thus different heightelectrodes, as illustrated by fingers 40′ and 42′ in FIG. 6, to obtainasymmetric electric fields, such as the fields 56. In one embodiment ofthe invention, the fixed fingers 40′ have a height that is approximately0.3 μm greater than the height of the movable fingers 42′. Thisdifference in the effective heights of adjacent electrodes produces agreater net vertical force 54′, thereby increasing the vertical motionof the movable structure. This motion is controllable by varying theapplied voltage, allowing the total amount of out-of-plane deflection ofthe micromirror platform 18 to be controlled within its operating range.

[0044] The micromirror 10 preferably is of micrometer scale, and itsdimensions can vary widely. For example, the backbone 44, the torsionalbar 30, and the fixed and movable fingers 40 and 42 may have widthsbetween about 0.5 μm and 3 μpm, heights of about 5 μm to 100 μm, with aspacing between adjacent fingers of about 2 μm to 4 μm. The thickness ofthe mirror platform 18 can range from 10 μm to 100 μm, with a mirrorsurface, such as the surface 60 illustrated in FIG. 3, of approximately320 μm×170 μm in one version of an experimental structure configured inaccordance with the invention. In this experimental structure, the sizeof the integral actuator 14 was approximately 382 μm×250 μm. Preferably,the mirror platform 60 is coated with a thin layer of reflectivealuminum film to produce the desired reflectivity, with the thickness ofthe platform being sufficient to maintain a smooth, flat surface evenduring motion of the micromirror 10.

[0045] In a second embodiment of the invention, the torsional motionmicromirror assembly 10 illustrated in FIGS. 1-4, is converted to avertical-motion micromirror 62, as illustrated in FIGS. 7-9, whereinelements common to the structure of FIG. 1 are similarly numbered. Inthis embodiment, the free end 64 of the backbone 44 (FIG. 1) isconnected to a second torsional beam, or support spring 70. Asillustrated, torsion beam 70 extends across cavity 32 in the substrate12 and is secured at opposite ends to wall 34. Z-direction motion, orvertical motion perpendicular to the plane of the actuator 14, isobtained in this embodiment by providing in the backbone 44 twostress-relieving hinges 72 and 74 spaced apart to divide the backboneinto three segments, generally indicated at 76, 78 and 80 in FIGS. 7Aand 8. The hinges allow backbone segments 76 and 80 to pivot aboutrespective torsion bars 30 and 70 and allow expansion of the length ofthe backbone when the application of a suitable voltage across theinterdigitated fingers urges the central segment 78 to move vertically,in the direction of arrow 82 in FIG. 8. To accommodate the requiredexpansion of the length of backbone 44 during vertical (z-direction)motion of segment 78, each hinge incorporates a pair of spaced,parallel, high aspect ratio bars, illustrated at 84 and 86 for hinge 72and at 88 and 90 for hinge 74. Bars 84 and 86 are perpendicular to, andlie in the plane of, backbone 44 and are connected to each other atopposite ends by respective connector beams 92 and 94. Similarly, bars88 and 90 for hinge 74 are parallel to each other, lie in the plane ofbackbone 44, and are interconnected at their ends by bars 96 and 98.

[0046] The widths of inner hinge bars 86 and 90 are greater than thewidths of the corresponding outer hinge bars 84 and 88 so that the innerbars 86 and 90 do not bend or twist when the actuator moves the segment78 of the structure vertically. Inner bars 86 and 90 and end bars 92, 94and 96, 98 sized to be rigid to provide vertical motion of the mirrorplatform 18 and to keep the platform planar. The outer bars 84 and 88,on the other hand are thinner and will twist and bend as required toaccommodate expansion of the length of the backbone during the verticalmotion of the central segment 78. Lateral motion of the mirror platformand the actuator 14 is eliminated by the symmetry of the hinges 72 and74 and the torsion beams 30 and 70.

[0047] As previously indicated, a large number of the micromirrorassemblies 10 or 62 may be fabricated on a single substrate andinterconnected to form a large array of single crystal siliconmicromirrors, as diagrammatically illustrated in FIG. 10. In thisfigure, a 6×6 micromirror array is illustrated; however, it will beunderstood that other configurations may be utilized, taking intoaccount the need for providing electrical connections to each actuator,the need to electrically isolate the elements, and the need to provideadequate coverage of the array by the reflective mirror surfaces 60 inorder to provide a sufficiently high mirror fill factor. As the arraybecomes larger, a greater number of interconnects must be routed betweeneach array element, and this requires that a significant amount of spacemust be left between adjacent mirrors for the address lines.

[0048] The array 100 is made up of micromirror structures 102, which maybe either the pivotal motion micromirror assemblies 10 of FIGS. 1-4 orthe vertical motion micromirror assemblies 62 illustrated in FIGS. 7-9.Address lines generally indicated at 104 are provided for themicromirror structures in known manner, with electrical connections tothe devices being made using contact pads formed on the surroundingsubstrate. For an n×n array, 2 n² address lines are needed, with twoelectrical connections being provided to each actuator. As illustratedin FIG. 10, for example, the suspended structures; the is, the movablefingers, in rows 2 and 3 share common address lines so that theelectrical connection to the suspended structures in these rows is byway of address line 110. Similarly, the suspended structures in rows 4and 5 are electrically connected to line 112. Each actuator then has aunique electrical connection for the fixed portions of the actuatorstructures; that is, for the fixed fingers. Thus, the fixed structureaddress lines for the actuators in rows 1 and 2 are routed parallel toeach other, as indicated at 104 and 105; the address lines for the fixedstructures of rows 3 and 4 are parallel to each other, as indicated at106 and 107, and the fixed structure address lines for the actuators inrows 5 and 6 are parallel to each other, as indicated at 108 and 109.The lines from the first 3 columns are routed in the negativex-direction, while the lines from the other 3 columns are routed in thepositive x-direction.

[0049] The spacing between the adjacent rows, indicated by the arrow AA,which is the spacing between rows 3 and 4, for example, is largelydetermined by the number of required address lines. Accordingly, thetwo-direction routing illustrated in FIG. 10 reduces the requiredspacing and improves the mirror fill factor. It should be noted that thelines 110 and 112 serve the dual purpose of providing an electricalconnection to the array, and providing substrate anchors for thesuspended structures. These lines must be wide enough to ensure thatthey remain attached to the substrate throughout the processing steps,and thus may be on the order of 20 μm in width. These anchors providemechanical rigidity to the structure.

[0050] Maintenance of the planarity and structural rigidity of thenarrow beams which make up the address lines becomes important as thearray size increases. For example, the line width and the gap betweenthe address lines may be 1.5 μm and 1 μm, respectively. These addresslines are released from the substrate during the SCREAM fabricationprocess. For larger arrays, wider address lines must be used consistentwith maintaining the desired mirror fill factor. With careful alignment,a fill factor approaching 90% may be provided.

[0051] A process for producing two-depth, large motion actuators isillustrated in FIGS. 11-17, to which reference is now made. This processis based on the SCREAM single mask photolithography process utilizing astandard silicon wafer. While the process is designed to produce 100 μmdeep fingers and 22 μm deep springs, the process flow makes clear howthe heights of the springs and of the fingers can be scaled by choosingthe appropriate etching steps. As illustrated in FIG. 11, a 2 μm thickPECVD oxide layer 120 is deposited on a [100] P-type polished siliconwafer 122. This oxide thickness is required, since a typical selectivityof silicon to oxide in DRIE etching is 100 to 1. An additional 1 μm ofoxide is usually added to protect silicon structures during theisotropic silicon etch for releasing portions of the structure, andduring the required over-etch of the mask oxide during the floor oxideclearance steps to be described. A resist layer 124 is spun onto the topsurface of the oxide layer 120 and is photolithographically exposed inknown manner to produce a pattern generally indicated at 126.Thereafter, the wafers are descummed, using an oxygen barrel asher, tocompletely clear the resist from the exposed area. The pattern in thephotoresist layer is then transferred to the oxide film 120 using a CHF₃plasma in a magnetron reactive ion etcher, illustrated in FIG. 12.

[0052] A first deep silicon etch, indicated by arrows 127, through thepattern 126 is performed to produce etched trenches 128, illustrated inFIG. 13. This step defines the depth of spring structures such as thetorsion bars 30 and 70. The patterned silicon structure is thenconformally passivated with a PECVD oxide layer 130.

[0053] After clearing the oxide from the floors of the trenches 128using a CHF₃ plasma etch, a second deep silicon etch is carried out, asillustrated by arrows 132, to increase the depth of the trenches 128 by,for example, an additional 30μ below the sidewall oxide 130, asindicated at 134, making the total height of the silicon structures 136located under the mask 120 approximately 50 μm. A time-controlled SF₆plasma, indicated by arrows 138 in FIG. 15, etches the silicon waferunder the protective oxide layer 130. This produces cavities 140 whichundercut the silicon structures 136 to selectively release thosestructures having silicon cores which are sufficiently thin to becompletely undercut and released. Thus, for example, beam 142, which maybe a spring structure with a core width of about 2 μm, is released,whereas structures having a width of, for example, 4μ will not beundercut sufficiently to be released, as illustrated by structures 144,146 and 148. In the illustrated embodiment, the released spring may havea core with a height of approximately 22 μm, giving it a high aspectratio with a height of 22 μm and a width of 2 μm.

[0054] After this first release, a third silicon etch step, indicated byarrows 150 in FIG. 16, increases the depth of the trenches 128 by, forexample, an additional 50 μm to make the structures 144, 146 and 148approximately 100 μm in height. These structures may form the fingersand backbone of the actuator structure for the micromirror assembly, forexample. In order to release the movable fingers and the backbone,another conformal thin layer of PECVD oxide 156 is deposited and thefloor oxide is removed, using an anisotropic CHF₃ oxide etch (FIG. 17).A slightly longer over-etch is done in comparison to the first flooroxide etch because the aspect ratio of the fingers is higher, due to thedeeper silicon etching. The etch species has a more difficult timegetting into small areas, so the etch takes longer to remove theseproducts.

[0055] After the floor oxide etch, a 30 μm silicon extension etch,indicated by arrows 158, is performed to release the remaining siliconstructures, using anisotropic SF₆ etch. This final release step does notreduce the height of the silicon structures that have already beenreleased, because the prior PECVD oxide deposition is conformal enoughto deposit the oxide underneath the structure 142, for example, toprotect it from further etching, as illustrated in FIG. 17. Followingthe release of the structures 144, 146 and 148 by the SF₆ etch, analuminum layer 160 is conformally sputtered over the surfaces of thereleased structures and the adjacent substrate 112 for metallization. Itis noted that the released structures illustrated in FIG. 17 include aPECVD oxide overhang, shown for example at 162, which isolates the metallayer 160 from the silicon core 164 of the released structures.

[0056] As illustrated in FIG. 17, the depth (or height) of the fingerstructures 144, 146 and 148 can be varied by adjusting the width of therespective structures in the masking step. For example, finger structure144 is narrower than structures 146 and 148, and thus was undercut bythe etching process more quickly than the wider structures, resulting ina shorter structure. Thus, the relative heights of adjacent actuatorcomponents such as beams, springs, and fingers is controlled, in theillustrated process, by selection of their respective widths. Thesecomponents have a silicon core, and are covered by a silicon oxide layerwhich is electrically insulating so that the fingers can be metallizedto form electrodes.

[0057] An alternate, preferred process for fabricating the actuatorstructure for the micromirror assemblies in accordance with theembodiments of FIGS. 1-6 and 7-9 is illustrated in FIGS. 18-28, to whichreference is now made. In both the above-described process and thepresent process, the actuator structure is fabricated on a top surfaceof a wafer after the micromirror platform structure has been fabricatedon the bottom surface thereof. Accordingly, the steps for fabricatingthe mirror platform are first described.

[0058] As illustrated in FIG. 18, a polished single crystal siliconwafer 170, is used as a starting material. The wafer may be 180-250 μmthick, but in the illustrated embodiment it is a 250 μm thick [100]wafer, and preferably is polished on both sides. 1 μm thick electricallyinsulating PECVD oxide layers 172 and 174 are formed on both the top andbottom surfaces of wafer 170; thereafter, in one form of the invention,a metal film 176, which may be a 0.5 μm thick layer of aluminum, forexample, is sputtered on the mirror side of the wafer, in this case onthe bottom oxide layer 174. A photoresist layer 180 is then spun ontothe surface of aluminum film 176 and is patterned, at 182, to define amicromirror surface 184 and surrounding trenches 186, using standardphotolithography. The pattern 182 is transferred to the aluminum layer176 by a Cl₂ plasma etcher and thereafter the oxide layer 176 isselectively etched using CHF₃ plasma. During this processing, thesurface of the front of the wafer is protected by the oxide layer 172.

[0059] The oxide etch of layer 174 exposes the silicon substrate 170through the trenches 182 around the mask segment 184 which defines thelocation and shape of micromirror platform which includes the mirrorsurface. The exposed silicon is etched (arrows 187) to a depth of about70 μm, as illustrated at 186 in FIG. 20. These trenches may, forexample, be 19 μm wide. Although the fabrication of only a singlemicromirror is illustrated, it will be understood that an array of thesestructures may be fabricated in the bottom surface of the wafer, withadjacent mirrors being separated by trenches 186 and the spacing formetal lines. Following fabrication of the trenches 186, the resist layer180 is removed and the wafer is descummed.

[0060] Thereafter, the top surface oxide layer 172 is stripped, using aCHF₃ plasma etch. After a thorough surface cleaning, a new PECVD oxidelayer 190 (FIG. 21) is deposited on the front surface 171 of wafer 170.This is done to insure that the mask oxide for front surface processingis free of scratches or defects. A resist layer 192 is then spun ontothe layer 190 and a first top surface photolithographic mask ispositioned on the photoresist layer 192 and is carefully aligned to theback side mirror structure 184 using a conventional infrared alignersuch as an infrared aligner. At this point in the process, the actuatorfabrication steps described above with respect to FIGS. 11-17 may becarried out.

[0061] In the preferred, alternative process, a pattern 194 whichdefines the area where the fixed fingers of the micromirror actuator areto be located is then exposed, as indicated at 194 in FIG. 21. Thispattern is then transferred to the oxide layer 190 using a CHF₃ plasmato selectively etch the area where the fixed fingers of the actuatorstructure will be located. In this step the oxide layer 190 is etched toa depth of 0.30 μm, and this depth becomes the height difference betweenthe fixed and movable electrodes after metallization in the SCREAMfabrication process used to form the fingers. This reduced thicknessportion of oxide layer 190 may be in the form of one or more trenches inlayer 190, as illustrated at 196 in FIG. 22. The resist layer 192 isthen removed, the wafer is descummed, and a second resist layer 200 isapplied to the top surface of oxide layer 190 (FIG. 23). The resistlayer 200 is exposed, as indicated at 202, using a second top surfacemask to define the -entire actuator structure, with the pattern 202being aligned with the oxide trenches 196. In one example of thisstructure, the width of the silicon cores of the actuator comb fingerswas selected to be 2 μm, with the effective finger length being 75 μm.Either the fixed or the movable actuator fingers may be patterned withinthe oxide trenches 196, while the other fingers are patterned on theoxide layer outside trenches 196. Thereafter the pattern 202 istransferred into the oxide layer 190, as illustrated in FIG. 24.

[0062] A 20 μm deep silicon etch (arrow 203) is performed through theoxide pattern 202 into the underlying silicon substrate 170 to producetrenches 204, the resist layer 200 is removed, and the wafer isdescummed. (See FIG. 25). Thereafter, an electrically insulating 0.2 μmthick layer of PECVD oxide 206 is conformally deposited in the trenches204 and the oxide is removed from the floors of the trenches, using aCHF₃ plasma. The sidewalls of the trenches remain covered by the oxidelayer, as illustrated in FIG. 25. Thereafter, an extension etch, 35 μmdeep, for example, is performed to deepen the trenches 204 asillustrated at 208 in FIG. 25 to facilitate the release of movablestructures. This is followed by a release etch (arrows 209 in FIG. 26)using SF₆ plasma to release actuator fingers such as the fingers 210 and212. The etch is timed so that only the desired structures are released;the posts which connect the actuator to the mirror platform as well asany required substrate anchors are not released, but are sufficientlythick to avoid being completely undercut. Following the release of theactuator components, a thin layer of PECVD oxide is deposited, asillustrated at 214, to provide a layer 214′ at the bottoms of thetrenches 204 and in cavities 208 which will serve as an etch stop forthe through-the-wafer etch which is to follow.

[0063] In order to release the micromirror platform for motion withrespect to the substrate following fabrication of the actuator structureusing either of the foregoing processes, the wafer is then etched fromthe back side, as illustrated in FIG. 27, to extend the depth of trench186 to the oxide etch stop layer 214′. After the exposed silicon in thebottom of trench 186 is completely etched away, the etch stop layer isremoved, as illustrated in FIG. 28, using a CHF₃ plasma, and themicromirror assembly is free to move pivotally or vertically withrespect to the substrate, depending on the actuator configuration. It isnoted that the dimensions of the actuator structure remain unchangedduring the through-the-wafer etch.

[0064] Thereafter, a 0.3 μm thick aluminum layer is sputtered on theactuator structure, as illustrated at 220 in FIG. 28. Electricalisolation is provided by the oxide layer 214, with the overhang 222below the released structures preventing the aluminum from contactingthe underlying silicon in the undercut regions of cavity 208. A heightdifference between fingers 210 and 212 is produced, in the preferredprocess, by the selective oxide etch region at 196, described above,which produces a different oxide thickness than occurs in the remainderof layer 190. When the fingers are metallized, electrodes of differingheights are produced as illustrated in FIG. 28 at 210 and 212. As notedabove, either the fixed or the movable electrodes may have the greaterheight.

[0065] In tests, structures fabricated in accordance with the foregoingprocesses were found to produce uniform vertical motion across themirror platform utilizing either the z-motion structure of FIGS. 7-9 orthe pivotal motion structure of FIGS. 1-4. In both cases, the motion isdriven by the two-height comb finger configuration illustrated hereinwith a mirror surface that was measured to be optically flat and smooth.

[0066] The two-sided wafer processing described above has the advantageof permitting fabrication of a mirror surface at the beginning of theprocess, and protecting that surface from damage during the lateretching steps used to fabricate and release the platform and actuatorassembly. Although the process is described in terms of providing ametal reflective surface on the mirror platform, and protecting thatsurface with a mask layer during later processing, it will be understoodthat this is exemplary, and that other mirror surfaces may be providedon the platform. For example, an active quantum well structure such asthin film layers of materials such as GaAs, GaInP, GaN, and the like maybe grown on the polished wafer, prior to fabrication of the micromirrorassembly, with this surface being protected by suitable masks during theetching processes described above. Similarly, the wafer surface can belithographically grooved to provide optical gratings before the assemblyis fabricated.

[0067] Although the invention has been described in terms of preferredembodiments, it will be apparent that numerous modifications may be madewithout departing from the true spirit and scope thereof, as set forthin the following claims:

What is claimed is:
 1. A micromirror, comprising a substrate having athrough cavity; a micromirror platform fabricated from said substrateand located in said cavity; an actuator fabricated from said substrateand suspended in said cavity for motion with respect to said substrate;and at least one support connected to suspend said platform from saidactuator for motion with the actuator.
 2. The micromirror of claim 1 ,wherein said actuator is suspended for torsional motion of said platformwith respect to said substrate.
 3. The micromirror, of claim 2 , whereinsaid actuator is secured to a beam extending across said cavity.
 4. Themicromirror of claim 3 , wherein said beam is secured to a first end ofsaid actuator, a second end of said actuator being free for torsionalmotion about said beam.
 5. The micromirror of claim 4 , wherein saidactuator includes a backbone connected to said beam, and multipletransverse movable fingers on said backbone interdigitated with multiplestationary fingers on said substrate to form a comb-type drive for saidactuator.
 6. The micromirror of claim 1 , wherein said actuator includesa comb drive.
 7. The micromirror of claim 6 , wherein said comb driveincludes movable fingers on said actuator interdigitated with stationaryfingers on said substrate, said movable and stationary fingers havingdifferent heights.
 8. The micromirror of claim 6 , wherein said combdrive is asymmetrical.
 9. The micromirror of claim 1 , wherein saidactuator is suspended for vertical motion of said mirror platform withrespect to said substrate.
 10. The micromirror of claim 9 , wherein saidactuator is connected between two spaced beams extending across saidcavity.
 11. The micromirror of claim 10 , wherein said actuator includesa backbone connected between said beams, and multiple transverse movablefingers on said backbone interdigitated with multiple stationary fingerson said substrate to form a comb-type drive for said actuator.
 12. Themicromirror of claim 11 , wherein said backbone is expandable.
 13. Themicromirror of claim 10 , wherein said actuator includes an expandablebackbone connected between said beams, and an asymmetric drive formoving said backbone vertically with respect to said substrate.
 14. Themicromirror of claim 10 , wherein said actuator includes a segmentedbackbone connected between said spaced beams.
 15. The micromirror ofclaim 14 , wherein said segmented backbone includes a first segmentsecured at a first end to a first one of said spaced beams, a secondsegment is secured at a first end to a second one of said spaced beams,and a third segment connected between said first and second segments.16. The micromirror of claim 15 , further including a first hingeconnecting said first segment to said third segment and a second hingeconnecting said second segment to said third segment.
 17. Themicromirror of claim 16 , further including a controllable drive mountedon said third segment for producing vertical motion of said thirdsegment.
 18. The micromirror of claim 17 , wherein said support for saidplatform is secured to said third segment of said backbone.
 19. Themicromirror of claim 17 , wherein each of said first and second hingesincludes first and second parallel bars interconnected to producetorsional motion of said first and second segments about said first andsecond beams, respectively, and concurrent changes in the length of saidbackbone upon vertical motion of said third segment.
 20. The micromirrorof claim 19 , wherein said controllable drive is an asymmetric combdrive.
 21. The micromirror of claim 19 , wherein said asymmetric combdrive includes interdigitated movable and stationary fingers havingdifferent effective heights.
 22. The micromirror of claim 21 , whereineach of said movable and stationary fingers incorporates a silicon corehaving an electrically insulating top and sidewall coating covered by ametal layer, to form an electrode.
 23. The micromirror of claim 22 ,wherein the thickness of the top electrically insulating coating on saidmovable fingers in different than the thickness of the top electricallyinsulating coating on said stationary fingers to produce said differenteffective finger heights.
 24. The micromirror of claim 22 , wherein saidsilicon core of said movable fingers has a different height than thesilicon core of said stationary fingers to produce said differenteffective finger heights.
 25. A method for fabrication of a movablemicromirror, comprising: coating top and bottom parallel surfaces of awafer with an electrically insulating layer; coating at least a portionof said bottom surface with a reflective surface; forming in said bottomsurface a trench surrounding a mirror platform including said reflectivesurface; forming in said top surface an actuator structure having amovable portion connected to said mirror structure; and releasing saidmirror platform from said wafer for motion with said actuator.
 26. Themethod of claim 25 , wherein forming said actuator includes fabricatingmultiple movable stationary interdigitated fingers having differenteffective heights for producing relative vertical motion of said movableand stationary fingers.
 27. The method of claim 26 , wherein fabricatingfingers having different effective heights includes fabricating saidmovable and stationary fingers with different core heights.
 28. Themethod of claim 26 , wherein fabricating fingers having differenteffective heights includes fabricating said movable and stationaryfingers with the same core heights and with top surface oxide layers ofdifferent thicknesses.
 29. The method of claim 26 , wherein forming saidactuator includes fabricating movable and stationary interdigitatedfingers having silicon cores with the same height, coating the tops ofthe cores of said movable and stationary fingers with differentthicknesses of an insulating layer; and metallizing said fingers toproduce interdigitated electrodes having different effective heights.30. The method of claim 26 , wherein forming said actuator includesfabricating movable and stationary interdigitated fingers having siliconcores of differing heights; coating said fingers with an electricallyinsulating layer; and metallizing said fingers to produce interdigitatedelectrodes having different effective heights.